Borrelia burgdorferi, the Lyme disease agent, is an extracellular pathogen that must adapt to both its tick vector and its vertebrate hosts. B. burgdorferi causes persistent infection that can last for the hosts' lifetime. The pathogenesis of B. burgdorferi depends on outer surface proteins that interact with the cells, extracellular matrix, and components of the hosts' immune system. I recently identified two novel fibronectin-binding proteins in B. burgdorferi named RevA and RevB. Both of these bacterial surface proteins are expressed during mammalian infection, yet repressed during colonization of vector ticks. I hypothesize that these unique fibronectin binding proteins are critical to the natural infectious cycle by helping B. burgdorferi interact with host tissues. I propose three Specific Aims: (1) analyze the roles of RevA and RevB during the B. burgdorferi infectious cycle; (2) define the mechanisms involved in RevA and RevB binding to fibronectin; (3) detail the mechanisms by which B. burgdorferi controls production of RevA and RevB. Knowledge gained from these studies will form the foundation for an R01 aimed at understanding how fibronectin-binding proteins contribute to B. burgdorferi virulence, thereby directing the development of novel therapeutic and preventative treatments for Lyme disease. Public Health Relevance: The Lyme disease agent, Borrelia burgdorferi, can infect humans for long periods of time and is frequently found associated with host connective tissues, and I identified two novel B. burgdorferi proteins that bind host fibronectin, a major component of host extracellular matrix (ECM). I hypothesize that fibronectin-binding proteins help B. burgdorferi interact with host tissues and are crucial for mammalian infection. The proposed studies will critically test this hypothesis, by examining the role(s) of fibronectin-binding proteins in B. burgdorferi infection processes, defining the mechanisms by which the borrelial proteins bind to their host ligand, and examining how the Lyme disease spirochete controls expression of these adhesins.